Interconnectivity in integrated circuits increases with decreasing size of functional elements and increasing complexity. To accommodate the growing demand of interconnections, complex configurations of conductors and insulators have been developed. Such configurations generally consist of multiple layers of metallic conductor lines embedded in multiple layers of insulators, which are fabricated from one or several low dielectric constant materials. The dielectric constant in such materials has a very important influence on the performance of the integrated circuit. Insulator materials having low dielectric constants (i.e. below 3.0) are especially desirable, because they typically allow faster signal propagation, reduce capacitive effects and cross talk between conductor lines, and lower voltages to drive integrated circuits.
One way of achieving low dielectric constants in the insulator material is to employ materials with inherently low dielectric constants. Generally, two different classes of low dielectric constant materials have been employed in recent years—inorganic oxides and organic polymers. Inorganic oxides, which may be applied by chemical vapor deposition or spin-on techniques, have dielectric constants between about 3 and 4, and have been widely used in interconnects with design rule larger than 0.25 μm. However, as the dimension of interconnects continue to shrink, materials with even lower dielectric constant become more desirable.
Since 1998 integrated circuits with 0.25 μm design rule have been in production, but will be superseded by the production of the 0.18 μm generation ICs in 1999, and materials having dielectric constants lower than 3.0 are needed immediately. As the trend to even smaller design rules continues, design rules smaller than 0.18 μm are being developed, and design rules of 0.07 μm and below can be expected in just a few generations, suggesting a strong need for dielectric materials with designed-in nanoporosity. Since air has a dielectric constant of about 1.0, a major goal is to reduce the dielectric constant of nanoporous materials down towards a theoretical limit of 1, and several methods are known in the art for fabricating nanoporous materials.
In some methods, the nanosized voids are generated by incorporation of hollow, nanosized spheres in the matrix material, whereby the nanosized spheres acts as a “void carriers”, which may or may not be removed from the matrix material. For example, U.S. Pat. No. 5,458,709 to Kamezaki et al., the inventors teach the use of hollow glass spheres in a material. However, the distribution of the glass spheres is typically difficult to control, and with increasing concentration of the glass spheres, the dielectric material loses flexibility and other desirable physico-chemical properties. Furthermore, glass spheres are generally larger than 20 nm, and are therefore not suitable for nanoporous materials where pores smaller than 2 nm are desired.
To produce pores with a size substantially smaller than glass spheres, Rostoker et al. describe in U.S. Pat. No. 5,744,399 the use of fullerenes as void carriers. Fullerenes are a naturally occurring form of carbon containing from 32 atoms to about 960 atoms, which is believed to have the structure of a spherical geodesic dome. The inventors mix a matrix material with fullerenes, and cure the mixture to fabricate a nanoporous dielectric, wherein the fullerenes may be removed from the cured matrix. Although the pores obtained in this manner are generally very uniform in size, homogeneous distribution of the void carriers still remains problematic.
In other methods, the nanosized voids are generated from a composition comprising a thermostable matrix and a thermolabile (thermally decomposable) portion, which is either separately added to the thermostable matrix material (physical blending approach), or built-in into the matrix material (chemical grafting approach). In general, the matrix material is first cured and crosslinked at a first temperature TXL to obtain a high TG matrix, then the temperature is raised to a second temperature TT (such that TT<TG) to thermolyze the thermolabile portion, and postcured at a third temperature (TC, with TC<TG) to form the desired nanoporous material having voids corresponding in size and position to the size and position of the thermolabile portion. Continued heating of the nanoporous material beyond TC will result in further annealing and stabilization of the nanoporous material.
In a physical blending approach, a thermostable matrix is blended with a thermolabile portion, the blended mixture is crosslinked, and the thermolabile portion thermolyzed. The advantage of this approach is that variations and modifications in the thermolabile portion and the thermostable matrix are readily achieved. However, the chemical nature of both the thermolabile portion and thermostable matrix generally determine the usable window among TXL, TT, and TG such that TXL<TT<TG, thereby significantly limiting the choice of available materials. Moreover, blending thermolabile and thermostable portions usually allows only poor control over pore size and pore distribution.
In the chemical grafting approach, a somewhat better control of pore size and pore distribution can be achieved when thermolabile portions and thermostable portions are incorporated into a single block copolymer. The block copolymer is first heated to crosslink the matrix, further heated to thermolyze the thermolabile blocks, and then cured to yield the nanoporous material. Alternatively, thermostable portions and thermostable portions carrying thermolabile portions can be mixed and polymerized to yield a copolymer, which is subsequently heated to thermolyze the thermolabile blocks. An example for this approach is shown in U.S. Pat. No. 5,776,990 to Hedrick et al. However, the synthesis of block polymers having thermostable and thermolabile portions is relatively difficult and labor intensive, therefore adding significant cost. Furthermore, as the amount of thermolabile portions (i.e. porosity) increases, the nanoporous materials tend to collapse more readily, thus limiting the total volume of voids that can be incorporated into the nanoporous material.
Although various methods are known in the art to introduce nanosized voids into low dielectric constant material, all, or almost all of them have one or more than one disadvantage. Thus, there is still a need to provide improved compositions and methods to introduce nanosized voids in dielectric material.